On top of that, that ‘useful’ could have been very limited. For example, shortening a half year drought by a week would not be worth anything if it just meant crops would die a week later.
You have to do some serious modeling on expected rainfall, the costs of reservoirs and water usage to figure out whether additional reservoirs are worth the investment.
It has been years since some of the large reservoirs were close to full. Where specifically do you believe new reservoirs should be built? And why do you do believe that they will capture water that keep from evaporating long enough to make a difference during the next drought cycle?
As it stands California’s reservoirs never seem to fill completely anymore. Adding more reservoirs won’t help store extra water if it simply isn’t falling from the sky.
You should not stop all the water from reaching the ocean in the first place, bad things happen when you do that. We are reaching the point where we just have enough water to keep the rivers flowing so we don't kill all the wildlife that exists in them, we're not adding significant amounts of water to storage.
You need to address the reason why they're empty. You said you know the system well, so do you mind explaining?